Polysaccharides are widely used as thickeners for water-borne coatings, particularly latex paints. Examples include hydroxyethyl cellulose (HEC), hydrophobically modified hydroxyethyl cellulose (HMHEC), ethylhydroxyethyl cellulose (EHEC), hydrophobically modified ethylhydroxyethyl cellulose (HMEHEC), carboxymethyl cellulose (CMC), methyl cellulose (MC), methylhydroxypropyl cellulose (MHPC), starch derivatives, guar derivatives such as hydroxyethyl guar, hydroxypropyl guar, locust bean gum, xanthan and alginates. Functions of the polysaccharide derivatives include adding viscosity to the paints, maintaining the viscosity during storage, and providing desired rheological properties during application of the paints.
Water-soluble cellulose ethers are widely used for thickening water-borne paints. They are typically made by chemically etherifying hydroxyl groups of cellulose with a wide variety of substituents, e.g. hydroxyethyl, hydroxypropyl and carboxymethyl. Cellulose is a linear polymer of anhydroglucose units which are connected by 1.fwdarw.4 .beta.-glycosidic linkages and has a molecular weight of about 500 to about 1,000,000.
The glycosidic linkages of cellulose and cellulose derivatives are susceptible to enzymatic hydrolysis, i.e., a water mediated process of cleaving the glycosidic bonds by cellulolytic enzymes (cellulases) leading to chain scission and molecular weight loss. Other polysaccharides and polysaccharide derivatives will undergo similar hydrolysis catalyzed by polysaccharide hydrolysis enzymes appropriate to the particular polysaccharide.
Many microorganisms release cellulase and/or other polysaccharide hydrolysis enzymes, and so latex paints thickened with cellulose derivatives or other polysaccharide derivatives and contaminated with these microorganisms may undergo viscosity loss upon storage due to molecular weight loss on the part of the polysaccharide thickeners. This effect is discussed by Springle in Journal of Oil and Colour Chemists' Association, vol. (71)4, pages 109-113. The viscosity loss of a paint is highly undesirable, because it makes the paint unsuitable for use, and causes a deterioration in the quality of the applied paint film.
One method that has been used to minimize enzymatic degradation of polysaccharide derivatives in paints is the incorporation of biocidal materials in the paints to destroy the microorganisms that produce the hydrolytic enzymes. In the case of cellulose derivatives, there also have been considerable efforts to make them resistant to enzymatic hydrolysis by chemical modification. One approach involves modification of cellulose by a high degree of substitution with placement of substituents uniformly along the chain by manipulation of the substitution reaction conditions. This approach is disclosed in U.S. Pat. Nos. 3,709,876; 3,769,247; 4,009,329 and 4,084,060, all of which are incorporated herein by reference in their entireties.
Canadian Patent No. 1,014,289 discloses water-based protective coating compositions comprising a particular hydroxyethyl cellulose (HEC) which imparts substantially increased biostability to the compositions. The maximum percentage of unsubstituted anhydroglucose units in the HEC is about 11 for molar substitution values ranging between 1.5 and 2.
Currently, available commercial cellulose derivatives are not sufficiently resistant to enzymatic attack. Hence there is a need to develop cellulose and other polysaccharide derivatives that will not undergo molecular weight loss in the presence of cellulase or other polysaccharide hydrolysis enzymes.
M. G. Wirick, in Journal of Polymer Science, Part A-1, vol. 6, pages 1705-1718 (1968) discusses the substitution pattern of hydroxyethyl cellulose and its relationship to its enzymatic degradation. In Journal of Polymer Science, Part A-1, vol. 6, pages 1965-1974 (1968), Wirick discusses the enzymatic degradation of carboxymethyl cellulose at several substitution levels.
U.S. Pat. No. 3,974,032 discloses a low D.E. starch hydrolysate having a narrow molecular weight distribution, containing less than about 20% by weight of starch oligosaccharides having a degree of polymerization greater than about 200 and having reduced enzyme susceptibility. The use of the low D.E. starch hydrolysates for making syrups having a solids content of 50-80% is disclosed.
U.S. Pat. Nos. 5,366,755, 5,525,368 and 5,569,483 disclose degradation products of polysaccharides or polysaccharide derivatives, and their use in foods. The preferred polysaccharide derivative is a cellulose derivative and the preferred mode of degradation is enzymatic degradation.
U.S. Pat. No. 5,566,759 teaches a method for reducing the viscosity of a cellulose containing fluid used during fracturing of oil and gas wells. An enzyme is used to degrade the polymer. Examples are presented utilizing hydroxyethyl cellulose and carboxymethylhydroxyethyl cellulose.
British patent application GB 2281073A describes reducing the molecular weight of a water-soluble cellulose ether by enzymatic hydrolysis. The viscosity of 2% aqueous solutions of the reduced molecular weight materials are from 1 to 10 cps. The use of the materials for the pigment coating of paper and for increasing the wet strength of paper is disclosed.
EP 382577 B relates to enzymatic hydrolysis of a cellulose derivative to form a mixture of oligomers having an average degree of polymerization in the range of 3 to 300 and a molecular weight of 500 to 100,000. Use of the oligomers in food is described.
Netherlands patent application NL 7413972 discloses natural gums such as gum arabic, gum tragacanth, alginic acid, ghatti gum, etc., that are modified by hydrolysis in the presence of a hydrolyzing agent which may be an enzyme. The products give oil-in-water emulsions, e.g. wax emulsions, with improved stability and lower viscosity.